Polyamide resins are widely used as engineering plastics having excellent mechanical strength such as impact resistance and friction/abrasion resistance as well as excellent heat resistance and oil resistance in the fields of automotive parts, electronic/electric equipment parts, office automation equipment parts, machine parts, construction materials/housing parts and the like, and recently have found increasingly wide application.
Many classes of polyamides including e.g., polyamide 6 and polyamide 66 are known, among which m-xylylene adipamide (hereinafter sometimes referred to as “MXD6 polyamide”) derived from m-xylylenediamine and adipic acid is positioned as a very excellent polyamide resin because it contains an aromatic ring in the main chain unlike polyamide 6 and polyamide 66 so that it has high rigidity, low water absorption and excellent oil resistance as well as a low shrinkage ratio during molding and causes little shrinkage or warp, which means that it is also suitable for precision molding. Thus, MXD6 polyamide has recently been more widely used as a molding material, especially extrusion molding material in various fields including parts of vehicles such as automobiles, general machine parts, precision machine parts, electronic/electric equipment parts, leisure/sports goods, civil engineering and construction materials, etc.
MXD6 polyamide has low water absorption as compared with other polyamide resins such as polyamide 66, but there is a need for molding materials having even lower water absorption to meet recent demanding requirements.
Lighter and stronger polyamide resin materials are also needed. A xylylene polyamide resin lighter than MXD6 polyamide and having lower water absorption includes a xylylene sebacamide resin derived from xylylenediamine and sebacic acid (hereinafter sometimes referred to as “XD10 polyamide”).
However, polyamide resins containing xylylenediamine as a structural unit are more liable to yellowing than polyamide 6 and the like because they tend to generate radicals at the benzylmethylene sites for structural reasons. The applicant proposed a method for preventing yellowing of MXD6 polyamide by adding a phosphorus antioxidant and an alkaline component in the polyamide resin (patent document 1). An anti-yellowing effect can be achieved by this method, but this method was difficult to use for some applications because the addition of a phosphorus antioxidant enough to prevent yellowing may increase costs or a phosphorus compound may deposit on a filter or the like during extrusion molding of a film.
As alternative to polyamide resins containing a xylylenediamine-derived unit as a structural unit, polyamide resins derived from bis(aminomethyl)cyclohexane having no benzylmethylene site and a dicarboxylic acid (hereinafter sometimes referred to as “BAC polyamides”) are expected to have high heat aging resistance. However, even BAC polyamides are not free from yellowing problems, and a polyamide resin derived from bis(aminomethyl)cyclohexane and sebacic acid (hereinafter sometimes referred to as “BAC 10 polyamide”) recently remarked as a BAC polyamide having especially low water absorption also had problems in yellowing and heat aging resistance.